The stretching-mode of OH- and OD- defects was studied using Raman scattering and Fourier transform infra-red absorption spectroscopy at 4 to 500K. In all of the materials, a <111> orientation of the OH- molecular axis was confirmed by the polarized Raman spectra and by stress-induced Fourier transform infra-red dichroism. The latter technique also yielded values for the disc-shaped elastic dipole tensor; aligning OH- parallel to an applied <111> uniaxial stress. Accurate band-shape analyses of low-temperature Fourier transform infra-red absorption and T2g Raman spectra gave, for low concentrations of OH- in LiF and KF, consistent values for their nearest-neighbor reorientational tunneling parameters (1.2 and 0.6/cm, respectively). For OH- in NaF, and for OD- in all three materials, a spectral sub-structure arising from tunnelling was undetectable in the narrow Lorentzian stretching-mode lines. The marked temperature broadening that occurred for the stretching-mode absorption and T2g Raman bands, and the smaller broadening of the A1g line, could be fitted by models for de-phasing of the transition by phonons; taking account of the presence of isotropic and anisotropic de-phasing components. In spite of the very different amplitudes of the OH- and OD- stretching-mode vibrations, their relative infra-red absorption and Raman intensities were very similar. The exception was the extremely weak oscillator strength of OD- absorption in KF.

Infrared Absorption and Raman Scattering of OH- and OD- Stretching-Mode Vibrations in Alkali Fluorides - Isolated Defects. E.Gustin, A.Bouwen, D.Schoemaker, C.P.An, F.Luty: Physical Review B, 2000, 61[6], 3989–99